Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are mostly frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is a particularly attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes for converting methane to liquid hydrocarbons involve first conversion of the methane to synthesis gas, a blend of H2 and CO. Production of synthesis gas is capital and energy intensive; therefore routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
Dehydroaromatization of methane via high-temperature reductive coupling has also been proposed as a route for upgrading methane into higher hydrocarbons, particularly ethylene, benzene and naphthalene. Thus, for example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole percent hydrogen and 50 mole percent methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5 and phosphorous-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1. The product effluent is said to include methane, hydrogen, at least 3 mole % C2 hydrocarbons and at least 5 mole % C6-C8 aromatic hydrocarbons. After condensation to remove the C4+ fraction, cryogenic techniques are proposed to separate the hydrogen and light hydrocarbons (methane, ethane, ethylene, etc.) in the product effluent.
U.S. Pat. No. 5,936,135 discloses a low temperature, non-oxidative process for the conversion of a lower alkane, such as methane or ethane, to aromatic hydrocarbons. In this process, the lower alkane is mixed with a higher olefin or paraffin, such as propylene or butene, and the mixture is contacted with a pretreated bifunctional pentasil zeolite catalyst, such as GaZSM-5, at a temperature of 300° C. to 600° C., a gas hourly space velocity of 1000 to 100000 cm3g−1hr−1 and a pressure of 1 to 5 atmosphere (100 to 500 kPa). Pretreatment of the catalyst involves contacting the catalyst with a mixture of hydrogen and steam at a temperature 400° C. to 800° C., a pressure of 1 to 5 atmosphere (100 to 500 kPa) and a gas hourly space velocity of at least 500 cm3g−1hr−1 for a period of at least 0.5 hour and then contacting the catalyst with air or oxygen at a temperature of 400° C. to 800° C., a gas hourly space velocity of at least 200 cm3g−1hr−1 and a pressure of 1 to 5 atmosphere (100 to 500 kPa) for a period of at least 0.2 hour.
U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. The addition of CO or CO2 to the feed is said to increase the yield of benzene and the stability of the catalyst.
U.S. Pat. No. 6,552,243 discloses a process for the non-oxidative aromatization of methane, in which a catalyst composition comprising a metal-loaded, crystalline aluminosilicate molecular sieve is initially activated by treatment with a mixture of hydrogen and a C2 to C4 alkane, preferably butane, and then the activated catalyst is contacted with a feed stream comprising at least 40 mole percent methane at a temperature of 600° C. to 800° C., a pressure of less than 5 atmosphere absolute (500 kPaa), and a weight hourly space velocity (WHSV) of 0.1 to 10 hr−1.
Russian Patent No. 2,135,441 discloses a process for converting methane to heavier hydrocarbons, in which the methane is mixed with at least 5 wt % of a C3+ hydrocarbon, such as benzene, and then contacted in a multi-stage reactor system with a catalyst comprising metallic platinum having a degree of oxidation greater than zero at a methane partial pressure of at least 0.05 MPa and a temperature of at least 440° C. Hydrogen generated in the process may be contacted with oxides of carbon to generate additional methane that, after removal of the co-produced water, can be added to the methane feed. The products of the methane conversion are a C2-C4 gaseous phase and a C5+ liquid phase but, according the Examples, there is little (less than 5 wt %) or no net increase in aromatic rings as compared with the feed.
Existing proposals for the conversion of methane to aromatic hydrocarbons suffer from a variety of problems that have limited their commercial potential. Oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, frequently require expensive oxygen generation facilities and produce large quantities of environmentally sensitive carbon oxides. On the other hand, existing reductive coupling techniques frequently have low selectivity to aromatics and may require expensive co-feeds to improve conversion and/or aromatics selectivity. Moreover, any reductive coupling process generates large quantities of hydrogen and so, for economic viability, requires a route for effective utilization of the hydrogen by-product. Since natural gas fields are frequently at remote locations, effective hydrogen utilization can present a substantial challenge.
A particular difficulty in using natural gas as an aromatics source concerns the fact that many natural gas fields around the world contain large quantities, sometimes in excess of 50%, of carbon dioxide. Not only is carbon dioxide a target of increasing governmental regulation because of its potential contribution to global climate change, but also any process which requires separation and disposal of large quantities of carbon dioxide from natural gas is likely to be economically prohibitive. In fact, some natural gas fields have such high carbon dioxide levels as to be currently considered economically unrecoverable.
There is therefore a need for an improved process for converting methane to aromatic hydrocarbons, particularly where the methane is present in a natural gas stream containing large quantities of carbon dioxide.